This invention relates to the positioning of spacer grids for nuclear fuel assemblies.
Typically, fuel assemblies comprise long cylindrical fuel rods, supported together in a bundle arrangement, and spacer grids, located at regular intervals along the length of the fuel rods. The fuel rods are spaced in an array which includes guide tubes for control rods and an instrumentation guide tube. The control rod guide tubes are open at their upper ends to allow control rods into the fuel assembly for the purposes of regulating the nuclear reaction. The instrumentation guide tube is open at its lower end to allow an instrumentation probe to be inserted into the fuel assembly to monitor the operating conditions of the fuel assembly.
Intermediate spacer grids (ISG) are generally comprised of slotted rectangular grid strips arranged to intersect in an egg crate manner forming cells through which the guide tubes and fuel rods are inserted. The spacer grids support the fuel assembly and maintain lateral spacing between the guide tubes and fuel rods. Spacer grid assemblies must maintain precise spacing between fuel rods in order to insure mechanical and thermal stability during operation of the fuel assembly. It is thus very important that spacer grids remain fixed during operation of the fuel assembly. For a discussion of the problems associated with improperly spaced fuel rods, see U.S. Pat. No. 4,294,660, which is incorporated herein by reference.
A number of means are used to prevent movement of intermediate spacer grids. One method for restraining an intermediate spacer grid from axial movement in a nuclear fuel assembly is shown in FIG. 1 and consists of a column of spacer sleeves 8 and inserts 9 placed concentric to the instrumentation tube in the center of the fuel assembly. The inserts are installed in the center cells of each intermediate spacer grid, and consist of a short length of tubing with a welded collar 10 at one end. The welded collar interfaces with the grid strips and prevents the intermediate spacer grid 11 from moving upward. The spacer sleeve below each intermediate spacer grid interfaces with the intermediate spacer grid to prevent it from moving downward.
The above discussed spacer sleeve to intermediate spacer grid interface below each grid is considerably weaker than the upward interface, thus potentially allowing the intermediate spacer grid to slide downwards if it becomes caught during routine fuel handling. Inspection for and repair of irradiated spacer grids that are displaced in such a manner tends to be very tedious and expensive.
Another example of a related prior art spacer grid retaining system is disclosed in U.S. Pat. No. 4,678,630.
Additional problems associated with prior art spacer grid systems include misalignment of the spacer grid restraint systems and spacer grids during installation which can lead to instability in the fuel assembly structure; bulky or complex restraint configurations which lead to increased pressure drop through the fuel assembly and unnecessary structural volume in the active fuel region; expense and time associated with fabricating and installing restraint systems which use welded connections to prevent longitudinal movement of intermediate spacer grids, as well as problems and costs associated with the failure of welds in these systems.